This invention relates to capacitive pressure sensors or capacitive differential pressure sensors.
Such a pressure sensor commonly comprises a substrate of ceramic, glass, or single-crystal material (e.g., sapphire) and a diaphragm of ceramic, glass, or single-crystal material (e.g., sapphire) which covers and is spaced from the substrate to form a sensing chamber between the diaphragm and the surface of the substrate facing the diaphragm.
The facing surfaces of the substrate and diaphragm are provided with electrodes, of which every two electrodes lying opposite each other form a capacitor, and which can be deposited by sputtering, for example.
When a pressure of a process medium acts on and deforms the diaphragm, which is elastic, the distance between the electrodes changes, which causes a proportional change in the capacitance of the capacitor. This change in capacitance is an electric signal representative of the pressure, which can be further processed in evaluation electronics.
U.S. Pat. No. 5,050,035, for example, discloses a capacitive pressure sensor comprising:
a substrate of alumina ceramic having a first major surface and a second major surface as well as a circumferential surface;
a first electrode of an electrically conductive material on the first major surface;
a first through connection from the first electrode through the substrate to the second major surface;
a diaphragm of alumina ceramic which
is attached to the substrate along a joint by a joining material, and
is covered, on a surface facing the first electrode, with a second electrode,
to which contact is made through the joint and via a second through connection from the joint to the second major surface,
the electrodes, the through connections, and the joining material being a resistive or conductive paste deposited by silk-screen printing.
To measure a difference of two pressures, referred to herein as a xe2x80x9cdifferential pressurexe2x80x9d, two sensing chambers are commonly used, one for each pressure, the sensing chambers being spatially and mechanically connected with one another and being provided with at least one capacitor each. In this manner, an electric signal can be produced which corresponds to the difference between a pressure acting on one of the sensing chambers and a pressure acting on the other sensing chamber.
Both the deposition of the electrodes by silk-screen printing and the sputter deposition require costly and complicated equipment, the sputtering requiring vacuum apparatus, for example. In addition, both techniques involve the use of masks with which the shapes of the electrodes are defined.
It is therefore an object of the invention to provide capacitive pressure sensors or capacitive differential pressure sensors wherein the electrodes are formed using a technique other than silk-screen printing or sputtering.
To attain this object, a first variant of the invention provides a capacitive pressure sensor comprising:
a substrate of ceramic, glass, or single-crystal material having a first major surface and a second major surface as well as a circumferential surface;
a plate-shaped first electrode of electrically conductive material
which is secured in a recess in the first major surface in a high-pressure-resistant and high-vacuum-tight manner by a first joining material;
a through connection from the first electrode through the substrate to the second major surface or the circumferential surface;
a diaphragm of ceramic, glass, or single-crystal material which
is attached to the substrate outside the recess along a joint by a second joining material and
either itself forms a second electrode
or is covered, on a surface facing the first electrode, with a second electrode,
to which contact is made through the joint.
To attain the above object, a second variant of the invention comprises a capacitive differential pressure sensor comprising:
a first substrate of ceramic, glass, or single-crystal material having a first major surface and a second major surface as well as a circumferential surface;
a plate-shaped first electrode of electrically conductive material
which is secured in a first recess in the first major surface in a high-pressure-resistant and high-vacuum-tight manner by a first joining material;
a first through connection from the first electrode through the first substrate to the second major surface or the circumferential surface;
a second substrate of ceramic, glass, or single-crystal material having a first major surface and a second major surface as well as a circumferential surface;
a plate-shaped second electrode of electrically conductive material
which is secured in a second recess in the first major surface of the second substrate in a high-pressure-resistant and high-vacuum-tight manner by the first joining material;
a second through connection from the second electrode through the second substrate to the second major surface or the circumferential surface of the second substrate;
a diaphragm of ceramic, glass, or single-crystal material which
is attached to the first substrate outside the first recess along a first joint by a second joining material,
is attached to the second substrate outside the second recess along a second joint by the second joining material,
either itself forms a second electrode
or is covered, on a surface facing the first electrode, with a third electrode,
to which contact is made through the first joint,
is covered, on a surface facing the second electrode, with a fourth electrode,
to which contact is made through the second joint.
To attain the above object, a third variant of the invention provides a capacitive differential pressure sensor comprising:
a substrate of ceramic, glass, or single-crystal material having a first major surface and a second major surface as well as a circumferential surface;
a plate-shaped first electrode of electrically conductive material
which is secured in a first recess in the first major surface in a high-pressure resistant and high-vacuum-tight manner by a first joining material;
a plate-shaped second electrode of electrically conductive material
which is secured in a second recess in the second major surface in a high-pressure resistant and high-vacuum-tight manner by the first joining material;
a first through connection from the first electrode through the substrate to the circumferential surface;
a second through connection from the second electrode through the substrate to the circumferential surface, said second through connection being separate from the first through connection;
a first diaphragm of ceramic, glass, or single-crystal material which
is attached to the substrate outside the first recess along a first joint by a second joining material and
either itself forms a third electrode
or is covered, on a surface facing the first electrode, with a third electrode,
to which contact is made through the first joint; and
a second diaphragm of ceramic, glass, or a single-crystal material which
is attached to the substrate outside the second recess along a second joint by a second joining material and
either itself forms a fourth electrode
or is covered, on a surface facing the second electrode, with a fourth electrode,
to which contact is made through the second joint.
In a first preferred embodiment of the invention, the plate-shaped electrode or the plate-shaped electrodes are made of a metal whose coefficient of thermal expansion is matched to that of the ceramic, glass, or single-crystal material.
In a second preferred embodiment of the invention, the plate-shaped electrode or the plate-shaped electrodes are made of an electrically conductive ceramic or an electrically conductive glass.
In another preferred embodiment of the invention, the electrically conductive ceramic is a cermet.
In a further preferred embodiment of the invention, the electrically conductive ceramic is a dispersive ceramic.
In still another preferred embodiment of the invention, the plate-shaped electrode or plate-shaped electrodes are made of a material selected from the group consisting of silicon carbide, titanium carbide, titanium nitride, titanium diboride, molybdenum disilicide, tungsten carbide, and zirconium carbide.
In a further preferred embodiment of the invention, the electrodes are sintered without the first joining material.
One advantage of the invention over the prior art mentioned above is that holes in the substrate (e.g., for through connections or for oil filling) no longer need be high-pressure-resistant and high-vacuum-tight. Furthermore, the diameters of such holes in the substrate can be greater than in the prior-art pressure sensors or differential pressure sensors, so that these holes are easier to form, for example by dry pressing at an unsintered green compact of the substrate.
The invention will become more apparent from the following description of embodiments when taken in conjunction with the accompanying drawings (not to scale). Like parts are designated by like reference characters throughout the figures; for convenience of illustration, reference characters which were already used are not repeated in subsequent figures. In the drawings: